This year the Presidio High School Upper Level Metals & Mechanics Class, AKA SkillsUSA Chapter 2538, built a Knucklehead Special for competition in the Projects class at SkillsUSA, formerly VICA, competition. My first motorcycle was a 1946 Knucklehead 74” that I owned for 25 years; I sold it during a medical crises in 1992 and have regretted parting with it ever since. I teach the class and sponsored the project, buying all of the parts and materials used in completing it.

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Two years ago we built a 1940 Flathead 80” Bobber that won a “Best of Show” trophy at the Texas SkillsUSA Championships. The bike belonged to me and I rode it around the Big Bend area for the next year. It was a great bike, but had some engine over-heating issues here in the desert that were due to inherent design flaws of the large displacement side-valve engine; it would not tolerate long-distance high-speed runs in the desert heat. Ultimately the front connecting rod blew at the big end with little warning, taking the crankcases and front cylinder along with it. I decided to sell the remains of the Flathead motor and look for a Knucklehead engine to install in the Bobber chassis; I located a basket-case Knuckle engine in Denton and picked it up last summer. The engine change required a new set of OHV Fatbob gas tanks to match as well, and I wanted to up-date the stock drum front brake to a disk to improve stopping. I started gathering all the needed parts and we set to work last August.

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Although the basket-case 1947 engine was fairly complete, I wanted to up-date many of the critical components for maximum strength and reliability. The bike is a custom, and I was not concerned with keeping it totally original or doing a restoration; I wanted old-time style with modern reliability. I sold or traded many of the original engine parts to collector friends of mine and we assembled a Knucklehead Special.

The crankshaft and cases are the foundation of the engine and we went with new S&S Cycle Super Stock crankcases with a stroker flywheel and crank assembly from Truitt & Osborn, running forged-steel H-beam rods. This set up uses late-model tapered roller bearings on the crankshaft, much stronger than the original flat roller bearings. We used Michael Brown Solutions Knucklehead replacement cylinders which have thicker walls than standard, allowing for a 3 ½” bore and giving us, with the 4 ¼” stroke on the flywheels, a total engine displacement of 80 cubic inches. The cylinders and cast 8.5:1 compression ratio standard 80” Shovelhead pistons were sent to Boretech in Ohio, who bored the cylinders and applied their silicone-carbide embedding process to the cylinder walls, said to provide up to 3 times the life of bare cast iron cylinders. I also had them Teflon-coat the piston skirts as well as ceramic-coating the piston crowns. Many other up-dates and improvements, such as a centrifugal-advance ignition timer, late-model CV Keihin carburetor, Andrew’s cam, S&S billet aluminum oil pump, Primo Belt Drive with a late-style clutch, and 12 volt electrics, were included, giving us a vintage-looking engine with modern power and reliability.

The challenge to incorporating parts from so many different models and manufacturers was that all fit and clearances had to be carefully checked to make sure that everything would be compatible. We received much good advice from folks on this forum as well as from Al at S&S. I supervised the job closely, double-checking everything, but the students did all of the work, from 3-angle valve job to checking the run-out on the flywheels and assembling the cases. They adapted the star-hub wheel to run a disk brake, machining all the needed spacers, and did the paint job. I am very proud of the high quality work they did.

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The style of the bike is that of a 1950’s era street racer. The look is deliberately rough-hewn and reminiscent of the racetrack. The guys who painted the scallops and hand-stripped the gas tanks of the original custom bikes and hotrods of the 1950’s had been painting similar designs on the engine nacelles of American fighters and bombers in Europe and the Pacific not too many years before.

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The bike scored a Blue Ribbon at the SkillsUSA District 2 meet in Odessa in February, and advanced to the Texas State SkillsUSA Championships in Corpus Christi in April, where it scored a rating of “Superior”. I think it was by far the coolest project in the exhibit hall.

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I was not willing to ride the bike on the street until after we had finished showing it, but after the State meet at the beginning of April we fired her up. Two priming kicks, switch on, and the engine started on the first kick! Unfortunately it was throwing clouds of black smoke out the tailpipe and rapidly fouled the plugs. When we had overhauled the H-D CV Keihin carburetor we had installed the richest jets in the kit, so we went back and leaned things out a little; restart, no change. After fiddling with the idle mixture needle a little and getting nowhere, we swapped the CV for a 38mm Bendix we had on hand for another project. The bike started and ran fine! I really wanted to run the CV because of its self-adjusting qualities in relation to altitude changes, which here in the Big Bend can vary from 1,800 feet above sea level to nearly 6,000 feet. I sat down with the H-D service manual and started checking the parts shown in the exploded drawing one by one. Imagine my surprise when I checked the passage for the low speed jet and found only a threaded hole! That would explain all the black smoke… I asked the student who had overhauled the carb about the jet, and he said that there hadn’t been any low speed jets in the overhaul kit and he hadn’t thought to check and see if there was one in the carb. The carb came from the take-off pile at Big Bend Cycle in Alpine and apparently had been robbed of its jet at some point. As it happened, the day we made this discovery, my wife, who also teaches at Presidio High School, was in Odessa for a workshop, so I got her to stop by the H-D dealership and pick us up a selection of jets. Next morning we screwed the richest of them into the carb, buttoned everything back up and varoom, she fired right up and settled into an even lope. The engine loads up a little at steady throttle and idle, puffing a little black smoke out the tailpipe, but I think I’m going to leave it a little rich until I’ve got 2,000 miles or so on the clock and am sure the rings are seated, then fine tune the CV a little

I’ve put just under 1,000 miles on the bike thus far, and it is running smooth and strong. We initially had an oil seep from the base of the rear exhaust valve tin, which has since sealed itself up; outside of that the motor has been oil-tight. The Teflon-coating on the piston skirts seems to be doing its job of reducing friction and keeping the cylinders cool. There has been no sign of any oil smoke from the top end out the tailpipe. We have had none of the problems that some other folks have experienced with the late model oil pump over-oiling the heads. As some of you may remember, we decided to set the pinion shaft bushing in the cam cover up for end oiling. Al, at S&S, advised me that it would be necessary to split the top end and crankshaft oil circuits as is done in the late model engines. This was easily accomplished by screwing an Allen screw, supplied with the S&S oil pump kit, into a pre-threaded passage in the crankcase behind the oil pump, and relocating the overhead oil line from the cam cover to the outlet supplied behind the rear lifter block on the S&S cases. I screwed a mini oil pressure gauge into the cam cover hole so I could keep an eye on the crankshaft oil pressure. Al suggested that it might be a good idea to restrict the overhead oil line to .060 at the crankcase fitting as well as where the “Y” line feeds into the rocker boxes, so I drilled three brass rivets .060 and inserted them between the line and the fittings at all three points. The rocker arms, fitted with plain bushings, seem to be getting plenty of oil and there have been no problems with leaks. The gauge on the cam cover, whose accuracy I am unsure of, shows 45 pounds of pressure when the engine is cold, rapidly dropping off to 10 pounds as the oil warms. I am running Mobil 1 V-Twin 20W-50W synthetic oil in an OEM horseshoe oil tank with a spin-on oil filter on the return line.

Altogether I am very pleased with the build and how the bike has performed to this point. I look forward to riding it long, far, and fast. I am especially proud of the high quality job that my Presidio High School students performed.

Randall......I remember well your very first post. I have awaited this day patiently. My hat is off to your students ! The lack of foresite to the missing carb part is a learning adventure. Small oversite, easy fix. The students instructor, you, require a double hats off !! What you have taught to those students, will remain in their lives, as the rock, all others stones are built upon, whether the task ahead of them is mechanical or other. I applaud you and the student body !! With deep respect and admiration, Pa

Thanks for the kind words, gentlemen, I'll be sure and pass them on to the boys. I rode the bike the twenty miles from my home in the mountains down into the valley for school this morning. She's running like a top!--- Randall

Your students are very lucky to have you as an instructor Randall. My welding class teacher would not let me bring in some custom pipes for an A10 Beezer a friend and I were working on. He said " No motorcycle parts in my classroom!" He was a good guy however and I learned a lot from him.

My metal shop teacher wouldn't let us do anything like that during class, but if you wanted to work on it after school, he'd give you a hand with it. That was 30- some years ago and I still use stuff i learned in that class, today. Congrats on giving your kids some knowledge they can use in their everyday lives.

Well, I had something a little distressing happen last Friday. I have just over 1,000 miles on the Knuckle and have been doing a break-in regime of constantly varying the speed and keeping below 65 mph. Friday afternoon I left school at 4 p.m. and ran a few errands around town, involving some stops, starts, and stop-and-go traffic. It was hot, 104 F. I left Presidio to ride home, 20 miles and a 2,500 foot climb. I got stuck behind 3 vehicles who were traveling just too slow for 4th gear and too fast for 3rd, so I decided to go ahead and pass them; this was on a pretty steep grade. I bumped her up to 70 and swung out to pass. As I came past the final vehicle the engine stumbled and missed a couple of beats. I went on past, pulled over on the shoulder and throttled down. The engine settled into a steady idle. I blipped the throttle a couple of times and watched the tailpipe. The was no gray smoke, no knocking, no hammering, everything sounded fine. I pulled back out and kept it under 65 mph the rest of the way home. I cranked her up several times over the weekend and she started right up and ran fine.

I think I probably got the motor hot enough to either vapor-lock the carb, I have no insulator block on the intake manifold other than the rubber seal ring on the CV spigot adapter, or perhaps the pistons got tight in the cylinders. Anyway, I ordered a Jagg 6-row Slimline oil cooler that mounts to the left down-tube of the frame; it should arrive tomorrow. I'll install it, then I'm planning to motorcycle through the Davis Mountains to San Solomon Spring at Balmorhea State Park. The Davis Mountains are higher then the Rio Grande valley, Presidio is about 1,800 feet above sea level, Marfa is 5,000 and Ft. Davis is 5,400, and there are a couple of 6,000 foot passes, so it should be some cooler. I ran a Lockhart oil cooler in the same position on my 1946 FL and had no over-heating problems. Hopefully, the Jagg will do the trick. I'll keep y'all posted.

Randall If it's that hot during the day, ride it at night. 104 during the day will kill a motor that is already running hot from the higher friction caused by new parts trying to meet each other and become friends. Nothing over 65 for the first 1000 miles. No riding at all in 95+ temps. I set my motors up pretty tight, but they last a long time when they are broke in right. Your description of putting it under load passing, riding in 104 temps, and blipping the throttle with only 1000 miles sent chills up my spine.

Well, Pa, sorry to give you chills, but if I didn't ride at all when the temps were over 95 F I wouldn't ride much in the Chihuahuan Desert. I don't ride at night if I can help it because of the deer and javalina (wild desert pigs) and because years as a welding instructor have pretty much wrecked my night vision. I've lived out here for years riding a Knuckle in the desert heat and they've held up well.

I thought Jerry might be right about the valves sticking. We used Rowe cast iron guides and Kibblewhite Black Diamond valves. I've had a problem with a sticking valve in a fresh Knucklehead in the past that I had fitted with bronze guides, which is one reason I don't use them anymore.

Compression is good and the bike fires right up after two priming kicks when cold. There is no oil smoke out the tailpipe and no piston slap (the pistons were set up at .002 clearance). I rode it in to school today and experienced missing as I climbed the steep grade out of the canyon where I live. The temperature was only 72 F and I'd gone less than a mile, so over-heating was not the problem. I pulled over at the top of the grade and the motor died as I coasted to a stop. I looked the bike over, checked the battery connections, and removed the left gas cap to check the fuel level; plenty of gas. I kicked the engine over a couple of times with the switch off to check the compression; plenty of compression. I switched on, kicked, and the bike fired immediately and settled into a smooth lope. I accellerated through the gears; plenty of power, smooth, no misses. I motor along at 55-65 mph for about 5 miles then the engine starts missing, looses power and dies. I coast to the side of the road and stop, look the bike over; nothing is visibly wrong. I removed the ignition timer cap and checked the point gap; its fine. I cleaned the points with a business card and closed the timer back up. Switch on, kick the bike through and she fires up on the third kick and settles into a smooth idle. No smoke, no knocks. I shifted into gear and pulled away; everything running nice and smooth. 5 miles and it starts missing again. I wiggled the ignition switch, a bad contact? No difference. I unscrewed the left-side gas cap enough to un-seat the seal and allow air into the tank; the missing eased. By then I was pulling into town and needed both hands for the controls, so I tightened the cap and rode on to school. In the lower gears the bike ran really smoothly, no missing, plenty of power, and settles into a nice idle at stops. ???

Well, the most obvious answer is that the right-side gas cap vent is partially or totally blocked and the problem is due to fuel starvation when the fatbobs vacuum lock. We used the aftermarket 4-gallon fatbobs so dispised by Plumber. These particular tanks use the late-style petcock and we fitted a Pingel high-flow petcock. The gascaps are the racheting type for the late-style screw-in bung; they seal well and don't leak. I'm going to check the vent passages on the right-side cap. There could also be some dirt in the floatbowl that gets sucked up at times of higher fuel demand. The Pingel has a very fine screen on it's pick-up tubes so I didn't fit an inline filter. I'm going to pull the floatbowl, and if there's any sign of dirt or water, I'll install an additional filter. I was about ready to do some rejetting anyway.

Thanks for your interest and I'll keep you posted. --- Randall

Last edited by Randall on Thu May 15, 2008 11:43 am, edited 1 time in total.

Randall,I have been fighting this same booger for 20 or more years. It has occurred on 3 very different Knuckles in my stable. I have a club brother who has also had this problem I went so far as to to take junk repro gas caps and vent them ala dirt bike gas cap. Read more of my attempts to settle this dilemma here http://www.knucklenutz.com/tech/viewtopic.php?t=1167Goodd luck and keep us informed.JKE

Aw Curt, c'mon man, don't be like that. I value your comments and am soliciting your opinion, or I wouldn't put myself out here like this. I could just lurk around and pick y'all's brains and never risk revealing my ignorance or screw-ups. I absolutely meant no offence or disrespect and am very sorry that I came across as a know-it-all. I would consider it a great loss if, because of my caustic aside, you were no longer willing to comment on my posts. I meant no offence to Pa either.

I have owned, at one time or another, three Knuckleheads, one of which I kept for 25 years, so I do have some experience with them, and most of that time I lived and rode in the desert Southwest, so I do have some experience with running old iron in high temps. Its not always ideal, but its what we have to deal with. I was just trying to make the point that, in the desert, its going to be hot. Your advice was sound and I was risking disaster by running a tight motor that way in that kind of heat.

The day I rode in to work, last Friday, and experienced the problem described, was in the low 70's when I left the house. I didn't know that it was going to hit 104 that afternoon, its still just mid-May, and if I'd had a choice I wouldn't have chosen that temperature to ride in, but I was 20 miles from home and the temperature was what it was. The temperature in the pass was probably 94, its 2,500 feet higher than Presidio and the temperature is usually 10 degrees cooler, but it was still hot. Instead of passing I probably should have dropped back, pulled over, and let those slow-moving vehicles clear the pass. In my defence, I had over 1,000 trouble-free miles on the bike, had had no over-heating problems to that point, it was running strong and I decided to goose it. I had taken some precautions, knowing that the heat was likely to be a problem, by having the piston skirts Teflon coated and coating the piston crowns with ceramic, still, I knew I was pushing it and if I'd seized and galled a piston it would have been nobody's fault but my own. The oil cooler arrived today and I'll install it for a little extra insurance as there is no practical way for me to avoid the desert heat entirely.

Since my post this morning I have checked the vent on the gas cap, and based on the somewhat subjective method of blowing through it, it passes air with about the same amount of effort as a spare cap I have requires, so I don't think vacuum-lock is the culprit. The floatbowl had a little very fine rust in it, probably from the filling station storage tank as the fatbobs were new clean steel that we prepped using the KREEM system etch and flush followed by tank liner coating. I'll go ahead and fit an in-line filter, but I don't think that's the problem. I've got a ride home, so I'm leaving the bike at school and in the morning I'm going to pull the dash and double-check all of the wiring connections and check the valve adjustment; maybe I've got a tight valve. Jerry's suggestion about sticking valves in a fresh motor also has merit, so I'm going to add a little Marvel Mystery Oil to the gas.

Fortunately, the engine seems to have taken no harm, and based on my experiences today, heat was not the culprit. I have read John's posts at Iron Motors Tech and the problems seem similar; its one of those frustrating intermittent things that could have any of a variety of causes. I am writing up my efforts to diagnose the problem as a way of modeling the systematic trouble-shooting process for my students, any interested readers of this forum, and to organize my own thoughts. Most importantly, the advice I have gotten here has always been valuable. There is a wealth of experience here and I appreciate any of you who take time to concern yourselves with what is, after all, my problem.

Heck........I'm not right all the time. Just most of the time .... Yeah....Right ! I luv the way I spelled swollen in my previous reply. I'm suprized anyone understood it. My Beezer had adaquate piston clearance but the ring gaps were to tight. I've seen the fuel starvation several times, intermittent coil failure and valve sticking, cause those symptoms as well. My first understanding of your problem was visualized as seisure, thus my reply. Pa

I wasn't trying to be snotty, but it did come off a bit like that, I guess. I've never owned a Knuck, but I've built lots of Harley motors over the years, and faced many problems.. Having said that, I've built far less motors than many on this board. What I was saying was that maybe it was time for me to shut the hell up before we got into a pissing contest. No offense intended.

I've installed the Jagg Slimline oil cooler on the left down tube of the frame. Hopefully this will prevent engine melt-down in our ridiculously hot climate. I re-routed the return line coming out of the spin-on oil filter I have mounted on the toolbox mount on the right side of the frame forward to the cooler and then to the return line on the oil tank. I purchased a cover to use in cool weather.

I had a similar cooler made by Lockhart installed on my '46 FL that I fitted with a Lockhart in-line thermostat that automatically bypassed the cooler until the oil temperature was above 180 degrees. I'd like to fit one on this bike, but Lockhart seems to have stopped making motorcycle oil coolers.

As luck would have it, it is raining here, for the first time in 4 months, and the temperature is 55 F; go figure...

Randall I would not rule out the gas cap vent issue too quick. Especially when you say it did it's thing when you got to higher elevation. Every one of my gas caps has added vent holes drilled. Also I have screwed up when creaming tanks and got the vent nipples between the tanks plugged up. That would do about the same thing.F

Randall, I've had the pilot jet on my CV get clogged up by a small speck of crud (from who knows where). I run a 45 pilot so the orafice is too small to pass any crud. I do have an extra filter in the gas line but will still run into this from time to time. Mike

I've had the PHS Knucklehead Special up on the stand in the school shop for the last week checking it over and looking for causes of the cutting out at speed that I experienced last week; I haven't found any obvious problems. I did rejet the carb and check the valve adjustment (rear exhaust a little tight), the ignition switch wiring, points and ignition timing. I haven't taken it back out on the highway this week as afternoon temperatures have been in the 102-104 F range (I'm taking Curt's most excellent advice). I'm staying in town late tomorrow for the school's awards program and will ride it home later in the evening after it cools off some. I'll get up in the morning this weekend while its cool and ride up and down the 10 miles of twisting canyon highway through the Chinati Mountains in front of my house and see how it does.

We've scheduled a photo shoot on the bike with a couple of pretty PHS Seniors next week to submit for a magazine article. While the bike was on the stand, I asked our art teacher, Laurie Holman, who did the nice blue pinstriping on the fatbobs, to jazz up the little "Step'n Out" skeleton stickers we'd put on the sides of the oil bag. Much of the bike was part of a basket-case Chihuahua City Police bike that I got out of Mexico some years back, so it seemed appropriate. My students spend waaayyy too much time across the river in Chihuahua, up to no good, so they got a big kick out of it![img][/img]

--- Randall

Last edited by Randall on Fri Jun 13, 2008 4:13 pm, edited 1 time in total.